Rapid heating of thick wall parison preforms

ABSTRACT

A parison preform is placed in a heating block so as to heat the parison from the outside by means of conductive heat, during which time a fluid such as air is circulated through the parison to impart heat from the inside.

United States Patent Inventor Charles L. Seefluth Bartlesville, Okla.

Appl. No. 783,657

Filed Dec. 13, 1968 Patented Jan. 12, 1971 Assignee Phillips PetroleumCompany a corporation of Delaware RAPID HEATING OF THICK WALL PARISONPREFORMS 10 Claims, 3 Drawing Figs.

U.S. Cl 263/7, 34/31 Int. Cl F27b 9/16 Field of Search 263/7, 52,

2; l8/(Digest); 34/31 [56] References Cited UNITED STATES PATENTS1,474,705 1 1/1923 Bluemel 263/7 1,786,142 12/1930 Wyman 263/7 2,515,8417/1950 Stuart 263/7 Primary Examiner-John J. Camby Attorney-Young andQuigg ABSTRACT: A parison preform is placed in a heating block so as toheat the parison from the outside by means of conductive heat, duringwhich time a fluid such as air is circulated through the parison toimpart heat from the inside.

PATEN-TED JAN 1 21971 3 554, 50s

INVENTOR. C.L.SEEFLUTH ATTORNEYS 3 1 RAPID HEATING or rmcx WALL PARISONmuaromus 7 BACKGROUND OF THE INVENTION I This invention relates to aprocess and apparatus for heating a preformed hollow parison.

While the blow molding art goes back over l-years, it has only been inthe last dozen years that the blow molding of hollow plastic articleshas'achieved significant commercial suc- 'cess. With this expandedutilization of blow molding techniques has come progressively morerigorous requirements from the purchasers of the finished product withregard to the appearance, durability, chemical resistance, low cost, andthe like of the product. 1

It is a well-known fact that many polymers which are particularlysuitable for blow molding suchas the olefin polymers, undergo thephenomenon known as orientation when stretched in the solid state at atemperature below the crystalline melt point. Since the orientationoccurs at a temperature just below the crystalline melt point while thepolymer is on a heating cycle, i't was early recognized that littleorientation could be effected in a conventional blow molding operationwherein a molten parison was extruded between mold halves and blown;first the temperature of the parison could not be controlled closeenough, and secondthe slight orientation achieved was predominantly inthe circumferential direction since at best only a minor amount ofstretching in the longitudinal direction occurred. One'method ofachieving the desired product is to extrude or mold a hollow parisonpreform, cool it to room temperature, carefully reheat is to just belowits crystalline melt point, and then transfer it to a molding stationwhere it is grasped at both ends, stretched longitudinally, thusimparting orientation in a longitudinal direction, and

thereafter placed quickly in a mold and expanded to conform to the shapeof-the mold by the introduction of fluid pressure into the interior ofthe parison, thus stretchingit circumferentially to impart orientationin a second direction. Such a greatly improved optical properties. I a

However, since it is necessary to heat substantially the entire parisonto a temperature within a rather narrowrange just 7 V below thecrystalline melting point in. order to take full advantage of theorientation phenomenon,-certain problems are inherent in such a process.Particularly with thicker wall parisons there is a problem of achievingeven rapid heating of theparisonpreform. 1- i With thicker walledparisons, the heating times in an air oven become undesirably high,generally being 20 minutes or more. The parisons. can be placed in aheating block, having holes corresponding to the outside diameter of theparison so. as to achieve more rapid heating through direct conductivetype heating, but this, too, presents certain problems. For one thing,as a practical matter, the heating block cannot be appreciably longerthan the length of theparison and thus there is inherent in such aheating method an "end effect" whereby the ends of the parison areheated less. Also, while the heating time is reduced by utilizing aheating block over that required in an air oven, even shorter heatingtimes are desired.

Also, heating the parison exclusively from the outside furthercomplicates the problem of sealing one end of the parison. The mosteconomical method of forming parison preforms is to extrude a continuouslength of tubing and cut this length of tubing into individual parisons,as opposed to injection molding a closed end parison preform. However,this necessitates sealing one end of the parison. It is apparent thatthe temperature at which maximum orientation occurs is below the optimumtemperature for achieving a good seal. This problem is furthercomplicated in heating of the parison in a heating block where the heatis imparted from the outside to the inside, since the portion of theparison which needs to be the hottest, the interior surface which is tobe sealed, receives the least heat. y 1

SUMMARY OF THE INVENTION In accordance with this invention, a parisonpreform is posi-- tioned within a heating block and heated fluid iscirculated inside it.

BRIEF DESCRIPTION or TI-IE DRAWlNGS FIG. 1 is a side elevation of aheating mechanism in accordance with the instant invention;

FIG. 2 is a section view along section lines 22 of FIG. I;

and 1 v 1 FIG. 3 is a section view along section lines 3-3 of FIG. 1.

DESCRIPTION OF THE. PREFERRED EMBODIMENTS The novel heating method ofthe instant invention can be used in the heating of hollow thermoplasticparison preforms made, for instance, of any orientable thermoplasticmaterial such as polymers of at least one l-olefin, said l-olefin having2 to 8 carbon atoms per mol'ecule polyvinyl chlorideand the like. It isespecially useful in the heating of parisons made from polymers ofl-olefins,-selected from the group consisting of ethylene, propylene,butene, and mixtures thereof, more particularly, polypropylene. Thesepolymers can be formed into hollow parison preforms and cooled to'atemperature below their crystalline freezing point. The presentlypreferred method of fabricating these preforms is simply to extrude acontinuous length of tubing which is severed into individual parisons ofthe desired'length.

These hollow parison preforms in the solid state are then heated bymeans of the. instant invention to a substantially. uniform temperatureof about 1 to about F. below the crystalline melting point'o f saidthermoplastic materials.

The instant invention "is of particular utility in heating parisonpreforms which are thereafter transferred to a moldingstation where theyare stretched longitudinally and expanded toconform to the shape .of amold by the application of internal pressure, thus imparting biaxialorientation to the finished product. This orientation process imparts asmuch as .a 20-fold increase in thetensile strength to the plasticmateri- The crystalline melt point can be determined by heating a smallpiece of the plastic under a polarizing microscope. The

. specimen is heated slowly to a temperature at which birefringencedisappears and this temperature is the crystalline melt point. H I v HThe heating block can be either stationary or movable.

Preferably, the heating block comprises a large metal-wheel having athickness slightly greater than the length of the parisons to be heatedwith at least one row of circles around the periphery of the wheel. Inorder to increase the capacity of a the wheel, two or move moreconcentric rows of holes can be present so that a parison after beinginserted at one point can travel approximately two revolutions of thewheel before being removed. In order to align the holes in the wheel,the axis carrying the wheel can he slid forward and backward.

Part or all of the wheel is then encased in an oven where heated air orthe like is circulated through the parison. Preferably, a wheel-typeheater is used with at least two separate ovens, so that the air can becirculated in one direction in one oven and in the opposite direction inthe other oven, so as to minimizethe end effect whereby the parison hasa natural tendency to be heated less at the ends which are near theedges of the wheel. Any heating means can be utilized to provide theheat in the ovens, one arrangement being a simthe air over the elementand circulate it through the hole in the wheel and thus over the innersurface of the parison.

Any gaseous fluid can be used in the heating ovens, air being preferredfor reasons of simplicity and economy. An inert gas could, of course, beused if desired; however, since it is the purpose of the invention toheat the parison up to a temperature just below the crystalline meltingpoint, it is apparent that problems of degradation are not severe atthese temperatures.

, Operating in accordance with this invention allows the benefits ofconductive-type heating which is much more rapid than heating which canbe effected in a simple air oven, and yet allows heating from boththeinside and outside so as to further speed up the heating process.

in this regard, it is noted that the instant invention is of particularutility in heating thicker walled parisons, that is parisons having awall thickness of greater than 100 mils, preferably l00200 mils, morepreferably 120-175 mils. It is difl'icult to heat thermoplastic parisonsto orientation temperature because of the relatively narrow temperaturelimits which are perrnisible. This is further complicated in theinstance of a thicker parison, and when viewed in terms of a commercialscale operation where long heating times 'to bring the parison up to thecritical temperature are precluded, the process becomes even moredifficult. Surprisingly, the particular combination of conductive heatfrom the outside and circulation of fluid such as air through theinterior of the parison, particularly from alternate'directions, effectsespecially uniform heating in a remarkably short time.

The heating block can be composed of any suitable structural material.Preferred materials are metals such as aluminum. It is essential thatthe portion of the heating block in contact with the parison be made ofa high thermoconductivity material such as aluminum, aluminum alloys,magnesium, or other metal. The heating wheel, itself, can be heated byany 7 conventional means, such as electrical resistance heating,

impinging a flame upon the outer surface of the wheel, or utilizing ahollow wheel having tubular inserts for carrying the parison andcirculating a fluid such as steam or heated oil through the hollowwheel. V i i Referring now to the drawings forming a part thereof inwhich like reference characters depict like parts in the various views,particularly FIG. 1, there is shown a circular heating block or wheel 12rotating about shaft 14. Around the periphery of wheel 12,concentric'with shaft 14, are a series of holes 16 adapted to receiveparisonsfThroughout most of its circumference, the peripheral'portion ofwheel 12 containing holes 16 passes through circulating; air ovens l8and 20. Preferably, there are at least two separate ovens so that theair can be circulated in one direction irt'one oven and in the otherdirection in the other oven to giveequal heating to both ends of course,three or more separateov'ens could be utilized. The temperature willgenerallybe identical in all of the ovens, although it is within thescope of the invention to utilize a higher temperature in the oven intowhich the parison is first conveyed after being inserted in the wheel,so as to speed up the initial heating of the parison.

Referring now to FIG. 2, there is seen the wheel and oven assembly ofFIG. 1 in cross section. Motors 22 and 24 are shown driving fans 26 and28 respectively. The motors can be located on the same side of thewheel, if desired, with provisions made to suck air over the heaters byusing areverse pitch fan or the like. Since the two ovens are identicalexcept for the provision for blowing the air over the parison in onedirection in one oven and the other direction in the other oven,detailed description is necessary of only one oven. In oven 18, forinstance, fan 26 blows circulating fluid such as air over electricallyheated resistance elements 30. The thus heated air is directed by meansof wall 32 and any accessory baffles which may be desired such as wall33 into hole 16 where said heated air fiows through the interior ofparison 34 from a first direction. As can be seen from a view of oven20, the air flows v of the parison. A single oven=could,be utilized,however, and V through the parison from a second direction, thusequalizing the heat imparted to each end. The entire apparatus issupported by frame 36. Shaft 14 is carried by bearings 38 and rotated bymeans not shown. Wheel 12 is heated by means of electrical cartridgeheaters 40.

Referring now to FIG. 3, there is shown means to insert parison whichcomprises guide '42 and pusher rod 44. Means to remove heated parisoncomprises guide block 46 and ex pandable prongs 48 carried by collar 50.Prongs 48 are moved axially forward, expanded to grip the heated parisonand thereafter retracted. A parisonis placed in guide 42 and pushed intohole 16 by means of pusher 44. The wheel then rotates forward anincrement to allow "removal of the next heated parison and insertion ofthe next cold parison.

ILLUSTRATIVE EMBODIMENT Polypropylene homopolymer, having a density of0.905 (ASTM D l50l-63T), a melt index of2 (ASTM D l238-62T.Condition'L), and a crystalline melting point of 340 F was extruded intotubing having an internal diameter of 0.85 inch and a wall thickness of0.15 inch-This tubing was cooled to room temperature and cut into 5 inchlengths. These 5 inch lengths of tubing are placed one at a time inholes corresponding in size to the parison so as to have direct metal topolymer contact in a rotating aluminum wheel such as is shown in FlG. 1having electrical resistance heating means. After being placed in thehole, the wheel is periodically advanced so as to move theparisonthrough a first oven where heated air is blown through the interior ofthe parison in a first direction. The periphery of the wheel then passesinto a second oven where heated air is blown through the parison in theopposite direction. The wheel continues to advance until the holecontaining the parison is back to the original starting point at whichpoint arnechanical prong is extended into the parison and expanded bymechanical means to contact the inner wall of the parison so as to gripthe parison from the inside. This mechanical prong is then retracted,pulling the heated parison from the wheel preparatory to theintroduction of a new parison by the feeder mechanism. The entire cyclerequires 4 minutes, the parison being heated substantially uniformly toorientation temperature of about 320 to 338 F. in this time.

For comparison, a second group of identical parisons were placed in the:same heater wheel under identical conditions ex cept that the ovenswere not used, all the heat being imparted by means of conductionthrough the outside, due to contact of the parisonwall with the heaterwheel. The wheel was rotated at the maximum speed possible to achieveheating of the parison to orientation temperature in l revolution. Thetime required was6minutes. As a further comparison, identicalparisons'wereheated in a circulating air oven under optimum conditionsto achieve rapid heating. The time required to heat the parisons toorientation temperature was 24 minutes.

While this invention'has been described in detail for the purpose ofillustration, it is not to be construed as limited thereby, but isintended to cover all changes and modifications within the spirit andscope thereof.

I claim:

1. A method of heating a tubular thermoplastic parison comprising:introducing said parison into a heating block; imparting heat to theoutside of said parison by conduction through direct contact with saidheating block; and passing heated gaseous fluid through the interior ofsaid tubular parison. I

2. A method according to claim 1 wherein said heating block is ametallic wheel rotating about an axis with at least one row of holesaround the periphery of said wheel concentric with said axis, said wheelbeing rotated so as to pass parisons after introduction into said wheelthrough at least one circulating air oven.

3. A method according to claim 2 wherein there are two circulating airovens and heated air is passed through the interior of said parisons ina first direction in one oven and is passed in the reverse direction inthe other oven.

4. A method according to claim 2 wherein said thermoplastic material isa polymer of a l-olefin selected from the group consisting of ethylene,propylene, butene, and mixtures thereof, said heating being controlledso as to heat the bulk of said thermoplastic material to within 1 to 50F. of the crystalline melting point of said thermoplastic material.

5. A method according to claim 4 wherein said thermoplastic material ispolypropylene. v

6. A method according to claim 4 wherein said parison has a wallthickness of greater than 100 mils.

7. A method according to claim 4 wherein said parison has a wallthickness of 120 to 175 mils.

8. Apparatus for heating a length of tubular material comprising incombination: a frame; a rotating wheel having an axis carried by saidframe, said wheel having at least one row of holes around the peripherythereof, said holes being arranged in a pattern concentric with saidaxis and adapted to receive said tubular material; means to heat saidwheel; at

least one oven disposed such that the periphery of said wheel containingsaid holes rotates through said oven throughout a portion of itscircumference; means to insert said lengths of tubular material intosaid holes; means to circulate heated air through the interior of saidlengths of material; and means to remove said lengths of tubularmaterial after a portion of said wheel containing said lengths has beenrotated through said at leastone oven.

9. Apparatus according to claim 8 wherein there are two ovens, saidmeans to circulate said heated air comprising a means to circulate saidair in a first direction through said lengths of material in one ovenand to circulate said air in the opposite direction in said second oven.

10. Apparatus according to claim 9 wherein the portion of said wheeladapted to receive said tubular material comprises aluminum.

1. A method of heating a tubular thermoplastic parison comprising:introducing said parison into a heating block; imparting heat to theoutside of said parison by conduction through direct contact with saidheating block; and passing heated gaseous fluid through the interior ofsaid tubular parison.
 2. A method according to claim 1 wherein saidheating block is a metallic wheel rotating about an axis with at leastone row of holes around the periphery of said wheel concentric with saidaxis, said wheel being rotated so as to pass parisons after introductioninto said wheel through at least one circulating air oven.
 3. A methodaccording to claim 2 wherein there are two circulating air ovens andheated air is passed through the interior of said parisons in a firstdirection in one oven and is passed in the reverse direction in theother oven.
 4. A method according to claim 2 wherein said thermoplasticmaterial is a polymer of a 1-olefin selected from the group consistingof ethylene, propylene, butene, and mixtures thereof, said heating beingcontrolled so as to heat the bulk of said thermoplastic material towithin 1* to 50* F. of the crystalline melting point of saidthermoplastic material.
 5. A method according to claim 4 wherein saidthermoplastic material is polypropylene.
 6. A method according to claim4 wherein said parison has a wall thickness of greater than 100 mils. 7.A method according to claim 4 wherein said parison has a wall thicknessof 120 to 175 mils.
 8. Apparatus for heating a length of tubularmaterial comprising in combination: a frame; a rotating wheel having anaxis carried by said frame, said wheel having at least one row of holesaround the periphery thereof, said holes being arranged in a patternconcentric with said axis and adapted to receive said tubular material;means to heat said wheel; at least one oven disposed such that theperiphery of said wheel containing said holes rotates through said oventhroughout a portion of its circumference; means to insert said lengthsof tubular material into said holes; means to circulate heated airthrough the interior of said lengths of material; and means to removesaid lengths of tubular material after a portion of said wheelcontaining said lengths has been rotated through said at least one oven.9. Apparatus according to claim 8 wherein there are two ovens, saidmeans to circulate said heated air comprising a means to circulate saidair in a first direction through said lengths of material in one ovenand to circulate said air in the opposite direction in said second oven.10. Apparatus according to claim 9 wherein the portion of said wheeladapted to receive said tubular material comprises aluminum.